Do I need to remove Pinion for differential swap (keeping stock gears)

Hey guys!

I have a 03 V6 Mustang that comes stock with a 7.5 inch open differential. I want to upgrade it to an LSD, but my questions is do I need to remove the pinion (i.e. reset pinion depth, etc) if I am re-using the stock gears?

The install doesn't seem too bad if I just pull the diff, switch them, and then set the backlash, but removing the pinion seems way more involved, and I don't have the tools to change races, press on bearings, etc.

Is it possible (or recommended) to try swapping a diff without planning on messing with the pinion?

As Joe suggested, it is a good practice to minimally check the pinion depth. There is a master housing depth for Ford housings that is used to determine proper pinion depth. If it was initially set correctly i.e. at the factory and no one has fiddled with it, it should still be good to go.

Here is a little but of additional information, that while not requested will none the less make the process a bit less daunting (hopefully) for you.

The Master Housing Depth approach to setting pinion depth uses a dimension from the differential bearing carrier center to the back side of the pinion gear surface that the rear pinion bearing registers against. Here is a drawing of the whole shebang;

The Master Housing Depth dimension (MHD) you want is the one in the red box. This is a listing of the MHD's for various popular Ford rear axles:

◉ 7.5" — 4.040"
◉ 8.0" — 4.000"
◉ 8.8" — 4.415"
◉ 9.0" — 4.375"

You want the 4.040" MHD dimension.

Now here is the significant part. If no one has meddled with the rear end since Ford set it up, barring any worn bearings that need replacing, the dimension should be unchanged from when Ford originally set it up. This means you can bolt a ring gear to the new LSD unit and put it in w/o a hitch. If the bearings are worn or damaged they will have to be replaced. When you replace the bearing(s) it will be necessary to reset the pinion depth to the correct MHD.

If you must reset the pinion depth then you will need a tool like this;

This particular one is for an 8.8" Ford. I don't believe the main cap bolt holes are in the same location for the 7.5" rear ends so measurements would need to be made off the 7.5" case and modifications to the drawing before having a machine shop make one for you. These plates are not sold commercially to my knowledge.

The plate rests against and is bolted to the main cap registers in the housing and you measure from the plate surface to the machined flat on the nose of the pinion. If the pinion does not have a machined flat on it's nose you need to take it to a machine shop and have them grind a flat that is perpendicular to the pinion's centerline. The set up plate is made 1" thick so the math works out easily, you just subtract 1.000 inch from the reading on the depth micrometer and add the pinion head thickness (PHT, see top drawing) to find your current housing dimension. Once you have that you add or subtract shims to get to the correct MHD.

Once again if the pinion has not been fiddled with since Ford put it in you will most likely be OK. If the pinion bearing(s) need to be serviced then you will need to reset the pinion depth as Joe has already indicated.

If I understand you correctly, I could remove the rear diff cover and check the pinion depth on the stock set with some gear paint. IF the pinon depth currently on the stock set up is correct, then I should be good to leave the pinion in place and just bolt in the new LSD and set the backlash. If the feathering pattern on the stock set up is screwed up, then I have a problem already with the stock pinion depth and need to fix it regardless of the LSD. I'm the single owner of the Mustang, and I don't think anyone has messed with the diff, so it sounds like I'm probably good to go with the current pinion depth. Either way I can go test it out before I invest in an LSD.

It is not possible to measure pinion depth with any of the gear marking compounds. The gear marking compounds will visually illustrate the quality of the meshing between the pinion and the ring gear. This approach is usually employed to optimize the gear meshing after proper pinion depth has been established. Gear marking compound can be used to bracket and through successive assembly and disassembly, close in on pinion depth but that is considerable work and usually beyond the skills set of the average enthusiast.

The best way to measure and/or set pinion depth to the MHD spec (quickly and accurately) is through the procedure I described above. It will require the use of a depth mic, some type of plate similar to the one I illustrated in post #4 above and the use of appropriate shims to position the pinion.

Thanks man! I got the new LSD all installed! Still need to do the clutch though. Right now I think I am going to work on redoing the shocks/struts/sway bars to improve the handling before I hit the track!

LSD backlash and wear pattern came in nearly identical to stock with the stock shims, and the car is running great right now with no diff noise! Didn't need to mess with the pinion at all. Removing/mounting the ring gear was kinda a nightmare though :-p.